This invention relates to photoelectric sensors using radiation pulses. In particular, this invention relates to such a photoelectric sensor incorporating means for effectively preventing misjudgments even where a noise pulse appears periodically and where the timing of the appearance of the noise is close to that of the timing of the sampling.
It has been known to use such sensors using radiation pulses to detect in a non-contacting manner the presence or absence of a target object, its distance and its shape. Examples of radiation pulses include light pulses, supersonic wave pulses, microwave pulses and impulse-type electromagnetic waves.
In general, such a sensor using radiation pulses includes a transmitting device for transmitting radiation pulses to a target area (also called a light emitter or a wave transmitter) and a receiving device for receiving radiation pulses coming from the target area (also called a light receiver or a wave receiver). Such a sensor may be of a transmission type or a reflection type. Radiation pulses transmitted from a sensor of the transmission type fail to reach its receiver if screened by a target object to be detected. Radiation pulses transmitted from a sensor of the reflection type reach the receiver by being reflected by the target object.
The transmitter and the receiver devices may be contained in one container or in separate housings. The type with the devices integrated within one container is advantageous in that the two devices can be easily correlated (or synchronized). Many sensors of the reflection type and many sensors of the transmission type with separate heads (such as those using optical fibers) are formed with the transmitter and receiver devices integrated. Many sensors of the transmission type of other kinds are formed with the transmitter and receiver devices separated.
Where a sensor using radiation pulses is set, there is usually all kinds of noise such as light, sound and electromagnetic waves, in addition to the regular radiation pulses. Affected by such noise of all kinds, noise pulses appear on the output line passing through the coupling capacitor of a detection converter (such as photoelectric converting element, ultrasonic-electrical converting element, magnetic-electric converting element, etc.) in the receiving device either through the converter or through the power line. Some noise pulses will appear periodically while some will appear randomly.
Measures of various kinds have been taken for preventing erroneous operations of the receiving device caused by noise pulse. One example of such measures depends on the technology of synchronized wave detection. With a sensor using this technology, the transmission of pulses from the transmitting device and the sampling by the receiving device are synchronized. Another method is based on the continuous nature of the received array of pulses. The output from a sensor adopting this technology will not be switched on unless more than a specified number of pulses are received continuously. Once the sensor output is switched on, it is not switched off unless a specified number of continuous pulses fail to be received. In other words, there is a hysteresis characteristic at the points of switching on and off. Still another method of preventing errors is to use both the synchronization technology and the pulse array. By this method, noise pulses which are off the timing of sampling are eliminated firstly by the synchronization technology and then the pulse row method is used next to eliminated the noise pulses which happened to coincide with the sampling timing.
These prior art methods are relatively effective against randomly appearing noise pulses but are nearly ineffective against noise pulses which appear periodically especially if the timing of their appearance coincides with the sampling timing. Such a situation frequently appears when a photoelectric sensor is set inside a factory or a storehouse with a fluorescent lamp of either the common frequency type or the inverter type or when a sensor of various kinds using radiation pulses is set inside a factory where a welding machine or a supersonic washing machine generating periodic electromagnetic noise is present. Since the timing of sampling for such a sensor is restricted in view of the response time required of the sensor, it cannot be freely changed to avoid the noise pulse.
It is therefore an object of this invention to provide a photoelectric sensor using radiation pulse which can function correctly even in the environment where noise pulse appears periodically and the timing of its appearance coincides with the sampling timing of the sensor.
It is another object of this invention to develop various technologies for such a sensor.
A sensor embodying this invention may be characterized not only as comprising an emitting device for emitting radiation pulses repeatedly and a receiving device for receiving these radiation pulses, but also wherein the receiving device includes converting means for converting the received radiation pulses into electrical pulses, wherein the receiving device contains pulse judging means for judging whether an electrical pulse appearing on the output line of the converting means is a true electrical pulse caused by receiving the radiation pulse emitted from the emitting device or a false electrical pulse caused by noise on the basis of a known waveform characteristic (or characteristics) of the true electrical pulse, and wherein the receiving device generates an output signal on the basis of result of judging by this pulse judging means. With a sensor thus structured, it is possible to distinguish between true and false electrical pulses not only when the noise pulse appears randomly but also if the noise pulse is generated periodically with the timing of generation coinciding with the timing of sampling.
In the above, the xe2x80x9cknown waveform characteristicxe2x80x9d of a true electrical pulse may be selected as values of the waveform at two or more standard points in time such as times corresponding to peaks of the waveform. It is preferable that these peaks include both peaks with positive and negative polarities. Alternatively, the known wave characteristic may be a value of the waveform obtained by differentiating the true electrical pulse which appeared.
The aforementioned pulse judging means for distinguishing between true and false electrical pulses may comprise two or more comparators each for comparing signal level appearing on the output line of the converting means with a standard value corresponding to the true electrical pulse, delay means for delaying an output of one of the comparators such that the outputs from the comparators are adjusted in time such that the standard times associated with the outputs will match, and logical calculation means for carrying out logical calculation on the time-adjusted outputs from the comparators.
A sensor according to another embodiment of the invention may be characterized also as comprising an emitting device for emitting radiation pulses repeatedly and a receiving device for receiving these radiation pulses, but its emitting device includes pulse transmitting means for transmitting the radiation pulses according to a specified bit pattern and its receiving device includes not only pulse judging means as explained above but also bit pattern judging means for making a comparison between the bit pattern of electrical pulses judged by the pulse judging means to be a true electrical pulse and a standard bit pattern and judging according to the result of this comparison whether radiation pulse was normally received. In the above, the xe2x80x9cstandard bit patternxe2x80x9d used by the bit pattern judging means for the comparison need not be the same as the xe2x80x9cspecified bit patternxe2x80x9d used by the emitting device, as long as they are correlated. With a sensor thus structured, distinction between true and false electrical pulses can be made even more accurately because the matching is made also in terms of their bit patterns.
It is preferable that the bit pattern judging means simultaneously compare the bit pattern of the electrical pulses appearing on the output line of the converting means with two or more preselected standard bit patterns having different phases, such that the sensor can depend on the results of these comparisons to judge whether or not the radiation pulses were normally received.
The expression xe2x80x9cdifferent phasesxe2x80x9d used above is intended to be interpreted broadly, including patterns with different bit arrangement (arrangement of bits representing transmission and bits representing non-transmission). For example, if there are two patterns A and B defined and if pulse transmission is carried out as Axe2x86x92Bxe2x86x92Axe2x86x92B . . . , both A+B and B+A may be used as standard bit patterns. If this is done, a matching is found whether the pattern appearing on the output line happens to start with A or with B. Thus, the bit pattern judgment can be effected more quickly and more dependably, that is, the response time of the sensor is improved.
It is also preferable that redundancy is provided in the matching by the bit pattern judging means such that even if some of the bits in the appearing pattern are different from what they normally should have been due, for example, to noise of some kind, a correct judgment can be made within a limit allowable by the redundancy. In general, it seldom happens that a bit which should indicate presence of pulse fails to so indicate. Situations may be more frequent where a bit which should indicate absence of pulse happens to indicate the presence of a pulse, but this kind of error can be effectively avoided by providing redundancy in the bit pattern judging means.
A photoelectric sensor according to still another embodiment of this invention may be characterized wherein the emitting device includes pulse transmitting means for transmitting light pulses according to an emission bit pattern based on arrangement of bits each indicating emission and non-emission of light, respectively, and wherein the receiving device includes bit generating means for generating light indicating bits each indicative of whether or not an electrical pulse appeared on the output line of the converting means and bit pattern judging means for making a comparison between a received bit pattern based on the light indicating bits and a standard bit pattern based on the emission bit pattern. The sensor judges from the result of this comparison whether light pulses emitted from the emitting device have been normally received and generates an output signal on the basis of the result of judging by said bit pattern judging means.
Here, too, the xe2x80x9cstandard bit patternxe2x80x9d need not be the same as the xe2x80x9cemission bit patternxe2x80x9d as long as they are correlated. A photoelectric sensor of this type can correctly distinguish between true and false electrical pulses even if there is a noise pulse appearing on the output line at the same timing as the sampling timing. In this application, it is preferable to preliminarily provide a plurality of such emission bit patterns each having a different arrangement of the bits indicating emission and non-emission of light such that judgment can be effected even more accurately. Means for generating emission bit patterns randomly may also be provided. In such a case, there must also be provide a means for communicating data between the light emitting and receiving devices. The emission bit pattern may arrange the bits indicating emission and non-emission of light in arrays.
It is preferable also in this example to preliminarily provide two or more standard bit patterns with different arrangements of bits indicating emission and non-emission of light such that the bit pattern judging means can make comparisons with the plurality of standard bit patterns. As explained above, if there are two patterns A and B defined and if pulse transmission is carried out according to pattern Axe2x86x92Bxe2x86x92Axe2x86x92B . . . , both A+B and B+A may be used as standard bit patterns. If this is done, a matching is found whether the pattern appearing on the output line happens to start with A or with B. Thus, the bit pattern judgment can be effected more quickly and more dependably, that is, the response time of the sensor is improved. It is also preferable to provide redundancy to the process of matching in this example.
It is preferable, furthermore, that at least two bits for indicating light emission be contained in a row in the emission bit pattern. A short noise such as so-called shot noise with duration less than 2 bits can also be identified if the bit pattern judging means is programmed to identify emission only if two successive bits indicate light emission. The number of bits and the emission bit pattern, as well as the length of bit in the emission bit pattern may be made variable.